The use of certain iodine-containing benzoic acid derivatives as X-ray contrast agents is well known. For example, U.S. Pat. No. 4,018,783 to Soulal discloses X-ray contrast agents which are of the structure ##STR2## wherein A is --CONHCH.sub.3 or --N(CH.sub.3)COCH.sub.3 and R is a phthalide group or lower alkyl optionally substituted with an aryl or dialkylamino group, or with a group of the formula EQU --OCOR.sub.1
wherein R.sub.1 is lower alkyl, except that when A is --CONHCH.sub.3, then R is not acetoxymethyl or pivaloyloxymethyl.
British Pat. No. 866,184 discloses triiodobenzoic acid derivatives of the structure ##STR3## wherein R.sup.1 is H or lower alkyl; R.sup.2 is H or lower alkanoyl; R.sup.3 is H or lower alkanoylamino.
U.S. Pat. No. 3,128,301 discloses 3,5-diacylamino-2,4,6-triiodobenzoic acid esters of the structure ##STR4## wherein R is lower alkyl, and R.sub.1 and R.sub.2 are lower alkanoyl.
U.S. Pat. No. 4,192,859 to Mackaness et al discloses the use of liposomes as carriers for contrast agents such as diatrizoic acid, sodium diatrizoate and other iodinated contrast agents.
Havron et al in "Radiopaque Liposomes: A Promising New Contrast Material for Computed Tomography of the Spleen", Radiology 140: 507-511, August, 1981, disclose radiopaque positively charged liposomes as carriers for diatrizoate meglumine and diatrizoate sodium (Renografin) for use in computed tomography of the spleen.
Ryan et al in "The Preparation and Characterization of Liposomes Containing X-Ray Contrast Agents," Biochemica et Biophysica Acta. 756 (1983), 106-110 (Elsevier Biomedical Press) disclose use of Renografin and Hypaque in liposomes in X-ray computed tomography.